Normally I get a sound similar to a chord when my Macbook Pro starts up. However, I get a sound like "do dit" before the normal startup sound. After some experimentation, I found that if I have a disk, CD or DVD, in the DVD drive, there is no "do dit" sound, only the normal startup sound. If the DVD drive is empty, I get both sounds. A searched of this forum and by Google yielded nothing. What is going on? How do I correct this annoying problem?